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Former anti-nuclear activists are coming out as pro-nuclear, warning that climate change is far more of a threat

California's Diablo Canyon is the state's last remaining nuclear plant. Over the last four decades, all the others have been shut down. And if activists like Linda Seeley have their way, Diablo Canyon will too.

"You cannot promote a technology that produces the most toxic substance on earth and also protect the public," she explained.

A truck parked outside a prayer protest against the Diablo Canyon Power Plant in California.America Tonight

Three years after the Fukushima Daiichi nuclear disaster, Seeley and Carole Hisasue are two of the activists from San Luis Obispo Mothers for Peace who gathered at the plant to pray for its closing. For them and many other activists across the country, there is only one lesson to be learned from Fukushima.

"I don't know how they're going to clean up Fukushima," Hisasue said. "This is the reality of nuclear power. You are sitting on top of a time bomb."

That ongoing disaster — an ocean away — has left many Americans skittish about nuclear power and further eroded support for a form of energy that had been popular with Americans since the 1973 oil crisis. Polls show that more than half of Americans oppose increasing nuclear energy and nearly 75 percent don't think the government should help build new plants.

But now a growing and unlikely movement of former anti-nuclear activists thinks the American public is wrong.

Coming out as pro-nuclear

Michael Shellenberger used to sound the alarm about nuclear energy.

"I grew up in an anti-nuclear family. I associated nuclear plants with nuclear weapons," he said. "And we just thought it was something sinister."

Today he believes that the world faces something else that's even more sinister — climate change — and that nuclear energy is necessary to fight it.

"I think the thing that really snapped us out of it was really just trying to figure out, How do you power a world of 7 billion while also dealing with climate change?" he said. "And you look at all the options available to you, and it's impossible to see how you stabilize emissions without using a lot of nuclear."

Patrick Moore, who helped found Greenpeace, in an ad promoting nuclear energy. He believes the rest of the environmental movement needs to "update its views."America Tonight

With a growing body of evidence that the global concentration of carbon dioxide is nearing the point of no return, he doesn't believe renewable energy is up to the task.

"It's hard to just imagine all that happening with wind turbines and solar panels. You've got to look for large quantities of energy in smaller amounts of space with a smaller footprint. So that's where I kind of came back to nuclear — but a bit grudgingly — a few years ago."

Shellengberger is just one of a small but influential group of former nuclear opponents, climate scientists and innovators who are coming out in favor of nuclear energy. And he believes the movement is growing fast.

In November four prominent climate scientists wrote an open letter urging those who influence environmental policy but who are opposed to nuclear power to reconsider. And on Monday the nonprofit group the Center for Climate and Energy Solutions, formerly the Pew Center on Global Climate Change, released a research paper charting the nuclear industry's decline and arguing that the United States' existing nuclear fleet is crucial to the transition to a low-carbon future.

"The loss of nuclear plants from the electricity grid would likely lead to millions of tons of additional carbon dioxide in the atmosphere each year," it warned. "This is a prospect the global climate cannot afford."

And while Fukushima has been a driving force for anti-nuclear activists, it has also mobilized the countermovement. Many of its members, like Shellengberger, believe the industry has learned valuable lessons from the disaster.

"Nuclear power plants actually got much safer after," he explained. "You saw a reaction by nuclear power operators all around the world to take a second look at earthquake and tsunami risks."

Next-generation nuclear

A time lapse of construction of a Westinghouse AP1000.

Nuclear defenders argue that there's a whole new generation of reactors built to be safer and more reliable than any technology we've seen before.

Jeff Benjamin, senior vice president of nuclear power plants at Westinghouse, the world’s leading nuclear power company, points to its AP1000, a reactor with a backup system that doesn't rely on electrical power to cool it.

Jeff Benjamin, senior vice president of nuclear power plants at Westinghouse.America Tonight

"We use the laws of nature, like gravity and condensation and convection to help mitigate the types of issues we had at Fukushima," he said.

Westinghouse is building four AP1000s in the U.S. — the first new reactor construction in decades. But even the new design requires a constant flow of water to the reactor core to prevent meltdown.

Now a new generation of nuclear environmentalists like Taylor Wilson are tackling the holy grail of reactor design: one that physically can't melt down and release radiation into the environment.

Taylor Wilson, the youngest person ever to create nuclear fusion, dreams of saving the world through nuclear energy.America Tonight

At the age of 14, Wilson became the youngest person ever to create nuclear fusion.

"I thought, 'Why could you not design a nuclear reactor that had no inclination to release radioactive material?'" he said. "So that's what I set out to do. I set out to save the world with nuclear power."

Now 19, Wilson believes the future of nuclear is in small-scale modular reactors that simply can't melt down like current ones.

Wilson's reactor uses molten salt as a coolant and is entirely sealed.

"You can actually drain the contents into a subcritical dump tank where nuclear reactions can't take place," he explained. "In no situation, properly engineered, should this release radiation into the environment."

The government recently gave half a billion dollars to companies developing similar modular technology. Wilson believes his design could be on the market in five years.

Beyond redemption

The nuclear industry has also turned to lobbyists to help it rebrand its image, releasing new ads, hiring three prominent former senators and a former chief of staff for President Barack Obama. It has formed the group Nuclear Matters, which has taken out print advertisements in national newspapers in order to raise awareness about nuclear energy.

But for many environmental activists, nuclear energy remains beyond redemption, and they consider the environmentalists who back it sellouts.

"When we produce something that is so toxic and so lethal, it's a crime," Seeley said. "It's a foul deed against the living Mother Earth. And it's wrong."